Quality of life control by selected methods of air exchange in a typical apartment building

: Air change rate is an important parameter for quantification of ventilation heat losses and also affects the indoor climate of buildings. Indoor air quality is significantly associated with ventilation. If air change isn't sufficient, trapped allergens, pollutants and irritants can degrade the indoor air quality and affect the well-being of a building's occupants. Many studies on ventilation and health have concluded that lower air change rates can have a negative effect on people’s health and low ventilation may result in an increase in allergic diseases. Quantification of air change rate is complicated, since it is affected by a number of parameters, of which the one of the most variable is the air-wind flow. This study aims to determination and comparison of values of the air change rate in two methods - by quantifying of aerodynamic coefficient C p = C pe - C pi – so called aerodynamic quantification of the building and the methodology based on experimental measurements of carbon dioxide in the selected reference room in apartment building.


Introduction
Buildings are currently built and modified to minimize energy losses and maximize efficiency. Efforts to reduce of ventilation heat loss reduce the air change rate. Many studies on ventilation and health have concluded that lower air change rates can have a negative effect on people's health and low ventilation may result in an increase in allergic diseases. [1,2,3]. Nowadays people spend up to 90% of their life indoors. Thus the quality of indoor air has received an increased attention in recent years. In a study [4] the authors try to establish the number of European residences that do not meet ventilation standards. They conclude that, up to 40% of European residences can be considered under ventilated. This number varies too by on the age of the building stock.
Air change rate -n -represents the amount of filtered air through leakage openings structures (joints, connections, ...) with natural ventilation, due to the action of the total differential pressure of air. Air exchange rate -n is a measure of the air volume added to or removed from a space in one hour, divided by the volume of the space and can be expressed as formula (1) (according to [6]):  Cp -total aerodynamic coefficient [-] w -wind speed [m/s].
The value of the total pressure difference Δpc is strongly influenced mainly primarily by the effects of the wind Δpw, which is very variable during the day and depends primarily on: the direction of the applied wind, the wind speed of the air flow above the ground and many other factors. Different investigators found a dependence on the square of the wind speed. A necessary input to formula (2) is the determination of the aerodynamic coefficient of total pressure Cp, which is currently the only problem in the field of physical quantification of natural ventilation of buildings [11,13,14].
For assessing of natural ventilation and for calculation of air change rate by using of simulation methods is it necessary the knowledge of distribution of pressure on the buildings facades and therefore is necessary aerodynamic quantification expressed by total aerodynamic coefficient Cp (-) takes into account the effects of variable wind with the parameters of the building. Knowledge of aerodynamic coefficients of external pressure Cpe and internal pressure Cpi is a basic prerequisite for aerodynamic quantification of buildings [11,13,15,16,17].

1. 1 External aerodynamic coefficient
The aerodynamic coefficient of external pressure is a dimensionless, highly variable quantity, which is influenced by a number of parameters -building geometry, details on the facade, position on the facade, degree of exposure, or. coverage, wind speed and wind direction [18,19]. Due to the number of parameters, it is not possible to take into account all parameters that affect it when determining the aerodynamic coefficient of external pressure.
Aerodynamic coefficients of external pressure can be expressed by: calculations according to national standards [20], experimental measurements in-situ [15,21,22], experimental measurements in the aerodynamic tunnel [23,24,25,26], simulations using CFD calculation software. Amin and Ahuja (2013) [26] performed a series of measurements on models of high-rise buildings with a rectangular floor plan in order to investigate the influence of the aspect ratio on the values of aerodynamic coefficients of external pressure. Similar measurements were performed by Amin and Ahuja (2011) [23] also on buildings with L and T-shaped floor plans. Between 2003 and 2007 a series of experimental measurements were performed at Tokyo Polytechnic University on 116 models of low-rise buildings and 22 high-rise models with rectangular floor plans and different ratios of width, length and height of the building and the results were summarized in aerodynamic databases (TPU Aerodynamic Database).
The aerodynamic coefficient of external pressure can be affected by a large number of parameters. Since the building modifies the air flow mainly by its shape, it is necessary to define the buildings under consideration geometrically in order to determine the aerodynamic coefficients of external pressure. At present, the spatial geometric classification is defined only for rectangular buildings of square and rectangular shape and for buildings with a circular floor plan. According to [11] we can divide buildings on the basis of height into three groups, namely low buildings (up to 15 m high), medium-sized buildings (15 m to 50 m high) and tall buildings (with height over 50 m).
For simple buildings -with a rectangular ground plan, the ratio height to width h: b = 3 and the height to length h: l = 2 -is a typical value of Cpe = 0.7 to 0.8 on the windward side and Cpe = -0.1 to -0.5 on the leeward and side walls [20,29]. The aerodynamic coefficients in the standards are the values applied in respect only strong winds and represent the maximum value for the façade. However, if the external aerodynamic coefficient unevenly, extreme value is significantly different from the average and at windward may be a difference of up to 50%.

1. Internal aerodynamic coefficient
In addition to external climatic factors, which are highly variable, pressure difference is affected by the air permeability of the peripheral structures. Façade shows a certain degree of the air permeability, which causes the changes of external and internal pressure. The wind load on the building envelope always depends on the pressure difference between the two surfaces of this structural surface and therefore both external and internal pressures need to be known. Research to estimate the internal pressures caused by wind has received much less attention than the measurement of external pressures, despite the fact that the internal pressure load of buildings contributes significantly to the overall load of the building envelope. Aynsley et al. [7] investigated the effect of wall porosity on internal pressures and found that the internal pressure is uniform and its value does not depend on the measurement site. Ginger [15] and Ginger et Letchford [27] studied external and internal pressures and their interrelationships along with the effect of a dominant opening on a low-rise building on a real scale. They concluded that the pressure inside the building depends on the distribution of external pressures and the location and size of the openings in the coat. The measured values of the internal pressure coefficients agreed with the values obtained by theoretical analysis of the steady flow through the opening.
Chen et al. [16] performed measurements on an acrylic model of a low-rise building with openings located in all four walls and the roof. They found that the angle of the acting wind is an important factor and hypothesized that in the case of multiple openings, the internal pressure value is affected by the opening located on the windward side and that the porosity of the building is not a major factor in changing internal pressure.
For engineering practice is very important to knowledge of the value of the internal aerodynamic coefficient, because it can cause result in increased values at leeward and lateral sides, because infiltration may cause alteration of aerodynamic coefficients of positive total pressure (pressure) to negative (suction) value [8]. Knowledge of aerodynamic coefficients of external pressure and internal pressure is a basic prerequisite for aerodynamic quantification of buildings using the aerodynamic coefficient of total pressure [11,28,15,16,17]. The approximate influence of the air permeability of the perimeter walls of a tall building of rectangular plan shape on the basic distribution of pressures and suction on its windward and leeward side is shown in Fig. 1 [11].

Figure 1
The approximate influence of air permeability of the perimeter walls of a high building with a rectangular floor plan on the basic distribution of pressures and suction on its windward and leeward side according [11] A -building with zero air permeability B -building with air permeability In the calculation of the internal aerodynamic coefficient Cpi it is necessary to know the modification of the building which affects changes of internal and external pressures [11]. Given that, in the current period there are no legislative requirements for quantification of air permeability of all dividing structures of buildings (partitions, doors, etc.), is it possible to deal with aerodynamic coefficients of internal pressure only for buildings without inner dividing by partitions [11].
The aerodynamic coefficient of internal pressure is a function of the parameter a [-], the value of which, since it is a dimensionless parameter, does not have to be quantified absolutely, but only proportionally [30] according to Equation ( To determine the parameter a [-], there are several assumptions by which the air permeability can be applied [11]. For its solution on the selected reference building, we assume that the air permeability of the perimeter walls is applied only by infill window constructions with different dimensions and with the same joint air permeability coefficient iLV [m 2 /(s.Pa n )]. Then applies the formula (5) [11]

2 Measurement of carbon dioxide concentration values
In order to evaluate actual air change rate, gas tracing dilution methods have been developed and standardized EN ISO 12569 [31]. Standard [31] describes among other method to the tracer gas concentration decay method which were used in this paper. According to [32,33,34,35,36], the tracer gas method may be used for determination of air change rate. The CO2 is used as a tracer gas in our case.
The method was used by Weining, et al. [37] in a study to determine the dependence of ventilation intensity by infiltration on wind speed. The research team Cui et al. [38] performed several experimental measurements in the laboratory in order to determine the error of measuring the air change rate in the building during cross-ventilation using the tracer gas decay method.
We can determine the concentration of CO2 by experimental measurements. In our case measurements were carried out predominantly during winter in one selected room. We conducted 24 measurements. From the measured data of CO2 concentration, it was possible to calculate the air change rate in room.
In the room was produced CO2 only by people. The continuing increase of CO2 concentration was caused from the presence of people. Throughout the time of stay in the room air exchange was caused by infiltration. If no person is present in the room, we assume a zero production of CO2. The tracer gas (CO2) concentration is monitored over time and the air change rate is determined from the rate of concentration decay. Therefore, the air change rate caused by the infiltration can be calculated from the function of decrease of CO2 concentration depending on time [34], where the influence of the CO2 concentration of the outdoor air Csup is considered by Laussmann and Helm [39]. The issue of airtightness of buildings is addressed also in paper [40].
The air change rate n caused by infiltration can be expressed as: Several contributions have been devoted to this issue, focused on natural air exchange, in the recent period [41,42,43,44]. In paper [45], the focus is on air exchange in the summer when considering energy savings. Posts [46,47,48,49] are devoted to the issue of air exchange in various types of buildings, the increase in CO2 and its impact on users.

Materials and methods
The subject of the paper is a living room -bedroom located in a flat in reference apartment building. The reference apartment building is located in the northern part of town Kosice, in eastern part of Slovakia (see Fig.3). The subject, goal and methodology of the research can be seen in Fig. 2. The aim is to determine and compare the values of air change rate in two methods-using quantifying of total aerodynamic coefficient Cp = Cpe -Cpi -taking into account the variable influence of the wind with the parameters of the building and accepting the air permeability of the façade and by the methodology based on experimental measurements of carbon dioxide in the selected reference building.
The methodology is focused on in situ measurements, calculations, confrontation of measured and calculated values and determination of the effects of selected parameters.
As already mentioned, in this article, the following methodology is applied: ▪ Calculation of air change rate without considering of openings ▪ Calculation of air change rate with considering of openings ▪ Calculation of air change rate on the basis of measured concentrations of carbon dioxide ▪ Comparison and verification of individual two methods

Reference room in selected apartment building
The selected reference apartment building is located in the centre of city Kosice, north (see Fig. 3 a,  b). Used reference room in the case study is located on the third floor of this selected building. a) b)

Figure 3 a) b) The situation of a case study
Views of the building from the exterior side can be seen in Fig. 4, as well as floorplan of the reference apartment and selected room can be seen in Fig. 5.

Description of the reference building
The reference building is situated (located) in the center of the city Kosice -North. It is a high-rise apartment building with 12 + 1 floors (total height 36.4 m), shaft type of building -a building with a vertical elevator shaft -position of the Neutral Pressure Plane is determined in the range of 1/2-2/3 a height of the building -24 m. The reference building has rectangular ground plan with dimensions: length: l=24.6 m, width b =12.1 m, height h = 36,4 m and 2 gable walls -see Fig. 4. According to [11] the reference building can be classified as: ▪ the medium height building with a height 15 m < h = 36.4 m < 50 m → buildings to 15 floors ▪ the geometry is of the ground plan l/b = ≈ 3 -the plate type building with spatial proportionality: 0,5 ≤ h/b = ≤ 1,5 and with surface area proportionality: 1,5 ≤ l/b = 2.3 ≤ 4,0 The building is insulated with a contact thermal insulation system and all apartments have the same types of windows.

Figure 4 External view of selected apartment building
The reference room is situated on 3nd floor -at height above ground approximately 8.4 m, oriented NW -315 ° -on windward wall. The reference room -bedroom is with internal dimensions 4 x 3.55 x 2.6 m and area of the room is 36.92 m 2 . The window system consists of a plastic frame, with an isolation binocular and a length of gaps l =12.1 m.

Measurement and description of the external climatic and internal parameters
External climatic parameters influencing the pressure difference pc are outdoor air temperature e, wind speed w and wind direction. On selected days was the wind speed measured at hydrometeorological station of 10 m above the open ground, but data on wind speed measured at hydrometeorological stations are not always identical to the actual speed characteristic of a particular site of urban form. Because is the reference building located in centre of city, values of wind speed measured in open terrain were reduced by [29] = . 10, where v10,met -wind speed measured at hydro-meteorological stations at 10 m height [m/s], k -coefficient -indicating the impact of terrain categories and the height above the ground [-]. Coefficient indicating the impact of terrain for reference building in centre of cities 10 m above the ground is k = 0,65. Internal climate parameters -indoor air temperature, internal air flow speed, internal air pressure and relative air humidity was measured using equipment TESTO 435 -4. For this purpose, a Testo 435-4 measuring instrument with a Testo 0632 sensor was used. Based on experimental measurements were assessed CO2 concentration of the indoor air. Measurements were carried out predominantly during winter. The CO2 concentration measurement range of the instrument is from 0 to 10,000 ppm, while the sensitivity is 1 ppm and the accuracy is ± 3%. The measurement range of temperature is from 0 °C to + 50 °C, with a sensitivity of 0.1 °C and accuracy of ± 0.3 °C. The relative humidity range is from 0 to + 100% RH, the instrument sensitivity is 0.1% RH and the accuracy is ± 1.8% RH.
To enable a mathematical description of the variation of CO2 concentration according to the measured data, it was important to ensure stable conditions during the measurements -room windows and doors were kept closed. A total of 24 experimental measurements were performed, where they were recorded -CO2 concentration, indoor air temperature, relative humidity and air pressure in the room.
The devices were placed close at a height of 1.0 m. During the measurement was person at least one metre and more away from the device, to prevent local influences on measurements.
The individual access of occupants was not allowed, entering or exiting was done simultaneously by the in the given time. In addition to the measured indoor parameters, hourly outdoor data of air temperature and wind speed were also recorded, since these have an impact on the air exchange rate caused by infiltration. External and internal parameters in selected days and hours are in Table 2.
During all experimental measurements were recorded: CO2 concentration, indoor air temperature, relative humidity and air pressure in the room. Course of one experimental measurement from 03.02.2019 No 15 in the apartment is shown in Fig. 6. On the Figure 6 is documented the course of indoor air parameters. The red arrow shows the selected section of decrease CO2 concentration of decrease. During this period, the room was closed and without persons. The CO2 concentration decreased only due to a leak in the building structure. From the record it is possible to observe, that the air pressure was constant, temperature difference a minimal and the relative humidity copied the course of the CO2 concentration. The detail of the course of CO2 concentration for the selected time period is documented in Figure 7.   It can be seen from Figure 7 that the maximum achieved CO2 concentration in the room was 1,728 ppm at 8:03. After the person leaves from the room and closing the door to the room the CO2 concentration began to decrease. The starting decline he was intense, but later at 8:30 was stabilized decline. The starting sharper decrease of CO2 concentration was caused by leaks in building structures and at the same time opening and closing the door, which was caused by the person leaving from the room to the next room. From the record it is possible to see that from 8:30, when the CO2 concentration was CIDA,S = 1,671 ppm, the decrease in CO2 concentration is regular. It can be assumed that from 8:30, the air change rate is ensured only by leaks in building structures. The CO2 concentration range in the outdoor air was from 392 to 428 ppm.
A total of 24 experimental measurements were carried. On some days were performed tree measurements and on some days only one measurement. All measurements were carried out during the normal use of the apartment so that the inhabitants were not limited. The only limitation was a time period when the person had to close the door to the room after leaving the room and was not allowed to enter it for about one hour.

Prediction of air change rate using quantifying of total aerodynamic coefficient Cp
Calculation of air change rate in the reference room were processed for selected days and hours with wind direction N, NW, NE -360 °, 315 °. The values of external aerodynamic coefficient for different wind direction are Cpe = +0.525 and Cpe = +0.35, internal aerodynamic coefficient Cpi was determined for building with two gable walls graphically according [28,29]. External and internal pressure act at the same time. The values of air change rate were calculated for building without considering the influence of openings Cp = Cpe and with considering the effect of openings Cp = Cpe -Cpi.
The values of air change rate for reference building with 2 gable walls for higher and lower wind speed are in the Fig. 8 and 9. In Fig. 8 and 9 shows and compares the value of air change rate without considering the effect of openings Cp = Cpe and with considering the effect of openings Cp = Cpe -Cpi for higher wind speeds from 4.4 m/s -10.3 m/s (Fig. 8) and for lower wind speed 1,1 m/s -4.0 m/s (Fig.9). From Figure 8 and 9 can be seen that the effect of the openings on the air change rate is significantly influenced by the wind speed. At higher wind speeds is the effect of the openings significant, see Fig. 8. The values of the air change rate n whit considering the influence of openings Cp = Cpe -Cpi are lower than without considering the openings Cp = Cpe, the difference between the values is in the range 0.05 -0.137 [1/h] (Fig. 9).

Determination of air change rate on the basis of measured values of carbon dioxide concentration
CO2 was produced only by people in the room. The increase of CO2 concentration was caused continuing by the presence of people. Throughout the time of stay in the room air exchange was caused by infiltration.
For each experiment, CO2 concentration measurements were made at time intervals of 1 minute. In order to calculate of air change rate, the duration of CO2 concentration decrease was considered as a multiple of several 1minutes time intervals. As an example, for measurements carried out on 03.02.2019, the first time interval was 1 minute and the last (31st time interval) was 31 minutes, resulting 31 calculated CO2 air change rates ( Figure 10).
From these results, the extreme values were excluded (the first four), and from the remainder of 27 values, the air change rate for that experiment was calculated as the arithmetic mean. A final value 0.11 [1/h] was obtained.    Based on the calculated differences listed in table 2A, 2B we conclude that the average margin of error is approximately 3.23%.

Discussion
This case study examines the effect of the wind direction and size and position of windows on the facade to interior air pressure. It points out the redistribution of these pressures and confronts the calculated results with experimentally measured values of carbon dioxide. It is used to find solutions in order to specify the intensity of air change, which significantly affects the thermal regime and comfort of the indoor environment. To determine the values of the of air change rate the calculation was used by aerodynamic quantification of buildings -account the influence of the wind with the parameters of the building and accepting the air permeability of the façade -and the actual measurements by means of the instrument, on the basis of which the experimental measurements of carbon dioxide was used. The results were evaluated and compared with each other. The values of the air change rate can be seen in Fig. 11, 12 where they are shown and compared values of air change rate for higher and lower wind speed.
At higher wind speeds v = 4.4 m/s -10.3 m/s (see Figure 11) is the effect of openings much more pronounced. At higher wind speeds v = 4.   m/s and 6.3 m/s, however, the outside air temperature was e = -3° C and e = +8.3° C.

Figure 13
The difference between values of air change rate taking into account the influence of openings Cp = Cpe -Cpi and the values based on experimental measurements of carbon dioxide for higher wind speed v =4.4 m/s -10.3 m/s However, these results also indicate that the value of air change rate is also at high wind speeds significantly lower than the value set by STN [5]  The aim of the work was to compare and verify the various methods and the above results clearly indicate that when accepting the air permeability of the facade of the building e.g. total aerodynamic coefficient Cp = Cpe -Cpi are values of air change rate comparable to the values obtained by calculation based on experimental measurements of carbon dioxide.

Conclusions
People spend more time at apartments than anywhere else, it is about 70%. Air exchange rate has a significant impact on energy consumptions and indoor quality. Proper use of natural ventilation can improve the indoor environment and reduce energy consumption. The windows enable natural ventilation and energy savings are ensured, are they obtained not only by increasing the thermal technical properties of the perimeter walls but also by the design and implementation of quality and tight windows. However, this often leads to a conflict between energy requirements and hygiene criteria. The air exchange rate is undersized and causes changes in humidity conditions up to the limit of hygienic requirements with possible subsequent adverse hygienic errors and the formation of mould. Therefore, it is necessary to ensure an increase in the intensity of air exchange through regular and intensive ventilation by apartment users or by means of microventilation.
The aim of the study was to evaluate the accuracy of the predictive value of determining the intensity of air change by comparing 2 methods -using quantifying of total aerodynamic coefficient Cp (aerodynamic quantification of buildings) and the method based on experimental measurements of carbon dioxide. This comparison can be generally applied for following conditions: single room with the exterior wall to the windward direction, no impact of interior restrictions to air movement, all leakage is due to window leakage, no air entering room from lower unit. Based on the calculations and measurements used in this study on different days (as shown in the tables and graphs), the results were compared and evaluated. As already mentioned, the results obtained by specifying the aerodynamic coefficient Cp = Cpe -Cpi taking into account the air permeability of the peripheral structures and the values based on experimental measurements of carbon dioxide are comparable and can be accepted.
At present, when manufacturers are trying to produce windows with almost zero joint air permeability, it is not possible to ensure natural air exchange with the windows closed. This problem must be solved by acknowledged micro-ventilation joints in the window construction. The eternal problem is to maintain a balance between hygiene and energy requirements. Hygienists, doctors would like a natural exchange of fresh air several times an hour, not only twice but three to four times. This is unacceptable for creators of artificial material environments, building architects who want to save energy for heating. When designing, they consider very small values of n (natural air exchange number) to predict low energy consumption for heating or cooling.
The current situation in the world, where infectious diseases (such as COVID-19) are spreading, people have to spend most of their time at home because it is forbidden to leave home. Children learn at home using computers in conjunction with the teacher via the Internet. With very tight windows, there is an increase in the amount and multiplication of bacteria in the indoor air. Therefore, the natural exchange of air for human health is very much needed. The whole process of such evaluation is based on very unstable methods, into which a number of unknowns enter. The building design process today requires completely different approaches than in the past. Everything leads to a certain virtual reality, simulation methods, where it is necessary to consider reference values for the calculation. Therefore, the value of air change when considering simulation tools requires that it be determined and verified by measurement. This study points to the possibilities of verifying the air change rate.
The results of measurements and calculations show that the values of the air change rate at both lower and higher wind speeds are below the standard level. This means that they differ significantly from the value for living rooms in residential buildings specified by the standard, which is n = 0.